I believe that German involvement would start to put more pressure on the confederacy, who would probably get France and the British empire involved. It may even start WWI a half century earlier.
What Germans?! There are no Germans in 1860/1861. Why would they get involved to begin with? Lincoln would thank them for their offer and turn them down. They giving assistance will complicate things regarding the Confederacy. The US doesn't need help in stamping out a rebellion will be what Washington says.
Germany, France and Britain would want to get involved with the ACW...Why? Hasn't this been done dozens of times before?
The British and French certainly considered intervening four times (the last attempt was awaiting first reading when news of the failure of Lees invasion of Pennsylvania reached England, and was quietly dropped).
The Prussians are hard to get in, they naturally favoured the CSA, so you need to change the dynamic somehow.